Because that "obsolete design" can actually track and shoot fast moving subjects, whereas this technology cannot.

... for now. I believe the issue with CDAF currently is the readout speed of the sensor. Already we're getting to 120 frames per second. As technology improves and that readout speed keeps increasing along with faster processor speeds, CDAF will get better and better.

PDAF is a very mature technology with some well-documented issues. CDAF eliminates those issues while introducing ones of its own. The difference is that the main issue with PDAF (keeping all elements of the imaging path in perfect alignment to ensure accuracy) is a problem inherent in its design, while the main issue with CDAF (speed, particularly tracking moving subjects) is one that will continue to be improved upon as technology progresses. Will CDAF ever be as good in all areas or better than PDAF? My crystal ball is hazy on that, but I would bet that it will eventually.